In a cellular communication system, a geographical region is divided into a number of cells each of which is served by base station. The base stations are interconnected by a fixed network, which can communicate data between the base stations. A mobile station is served via a radio communication link by the base station of the cell within which the mobile station is situated.
As a mobile station moves, it may move from the coverage of one base station to the coverage of another, i.e. from one cell to another. As the mobile station moves towards base station, it enters a region of overlapping coverage of two base stations and within this overlap region it changes to be supported by the new base station. As the mobile station moves further into the new cell, it continues to be supported by the new base station. This is known as a handover or handoff of a mobile station between cells.
A typical cellular communication system extends coverage over typically an entire country and comprises hundreds or even thousands of cells supporting thousands or even millions of mobile stations. Communication from a mobile station to a base station is known as uplink, and communication from a base station to a mobile station is known as downlink.
The fixed network interconnecting the base stations is operable to route data between any two base stations, thereby enabling a mobile station in a cell to communicate with a mobile station in any other cell. In addition, the fixed network comprises gateway functions for interconnecting to external networks such as the Public Switched Telephone Network (PSTN), thereby allowing mobile stations to communicate with landline telephones and other communication terminals connected by a landline. Furthermore, the fixed network comprises much of the functionality required for managing a conventional cellular communication network including functionality for routing data, admission control, resource allocation, subscriber billing, mobile station authentication etc.
Currently, the most ubiquitous cellular communication system is the 2nd generation communication system known as the Global System for Mobile communication (GSM). GSM uses a technology known as Time Division Multiple Access (TDMA) wherein user separation is achieved by dividing frequency carriers into 8 discrete time slots, which individually can be allocated to a user. A base station may be allocated a single carrier or a multiple of carriers. One carrier is used for a pilot signal which further contains broadcast information. This carrier is used by mobile stations for measuring of the signal level of transmissions from different base stations, and the obtained information is used for determining a suitable serving cell during initial access or handovers. Further description of the GSM TDMA communication system can be found in ‘The GSM System for Mobile Communications’ by Michel Mouly and Marie Bernadette Pautet, Bay Foreign Language Books, 1992, ISBN 2950719007.
Currently, 3rd generation systems are being rolled out to further enhance the communication services provided to mobile users. The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) wherein user separation is obtained by allocating different spreading and scrambling codes to different users on the same carrier frequency. The transmissions are spread by multiplication with the allocated codes thereby causing the signal to be spread over a wide bandwidth. At the receiver, the codes are used to de-spread the received signal thereby regenerating the original signal. Each base station has a code dedicated for a pilot and broadcast signal, and as for GSM this is used for measurements of multiple cells in order to determine a serving cell. An example of a communication system using this principle is the Universal Mobile Telecommunication System (UMTS), which is currently being deployed. Further description of CDMA and specifically of the Wideband CDMA (WCDMA) mode of UMTS can be found in ‘WCDMA for UMTS’, Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.
In a UMTS CDMA communication system, the communication network comprises a core network and a Radio Access Network (RAN). The core network is operable to route data from one part of the RAN to another, as well as interfacing with other communication systems. In addition, it performs many of the operation and management functions of a cellular communication system, such as billing. The RAN is operable to support wireless subscriber units over a radio link being part of the air interface. The wireless subscriber unit may be a mobile station, a communication terminal, a personal digital assistant, a laptop computer, an embedded communication processor or any communication element communicating over the air interface. The RAN comprises the base stations, which in UMTS are known as Node Bs, as well as Radio Network Controllers (RNC), which control the Node Bs and the communication over the air interface.
When a mobile station initiates a call or accesses a new cell for setting up a soft-handover leg in UMTS, it transmits an initial access message to the appropriate base station. This access message is known as a RACH (Random Access Channel) message. In UMTS, the RACH access mechanism uses a slotted ALOHA protocol wherein the random access channel is divided into discrete time slots that can be used for accessing the base station. The base station broadcasts timing information that the mobile station uses to synchronise to the time slots of the RACH channel. The mobile station transmits the RACH message by choosing a RACH time slot at random, and transmitting the RACH message in this time slot.
When the base station receives the RACH information message, it generates a data packet comprising the contained information and communicates it to a Radio Network Controller (RNC). The RNC is in charge of resource allocation for the air interface, and in response to the received information it proceeds to allocate communication resource to the originating mobile station or to reject the access request. The RNC communicates the required information back to the base station, which in response proceeds to setup and configure the communication link with the mobile station or to inform the mobile station of the rejection.
Effective radio access control is essential for an efficient resource utilisation and in particular for an efficient resource utilisation in a CDMA communication system where there is a balance between capacity and the performance of the system.
In conventional CDMA access control methods, a resource requirement associated with an access message is determined and the mobile station is admitted or rejected depending on this resource requirement. Specifically, a given requested service is typically known to require a specific signal to interference ratio and have a given bit rate. A required transmit power is determined that is guaranteed to result in required signal to interference ratio regardless of the specific operating conditions of the individual mobile station. Thus, each service is considered to have a corresponding nominal resource requirement which depends only on the service itself and not on the current or specific operating conditions. This resource requirement is used to decide whether to access or reject the mobile station.
However, this is clearly a very inaccurate approach as conditions for different mobile stations may vary considerably. Especially, for lower spreading factors, practical measurements show that a downlink transmit power may vary y between typically 1 and 10 W for the same service dependent. This variation may correspond to 20% or more of the total available transmit power in a cell. Clearly, admission decisions based on a conservative resource estimate will allow for acceptable performance of the admitted mobile stations but will result in many access requests being rejected which could have been adequately supported by the base stations. This will cause the cell to be under-loaded and will result in dropped calls and a reduced capacity of the communication system as a whole. However, using a less conservative estimate may allow more mobile stations but the base station may in some cases not be able to support all the admitted services resulting in degraded performance and possibly dropped calls for all the mobile stations supported. Thus, it is clear that current access control algorithms are suboptimal and result in degraded performance and capacity of the communication system.
Hence, an improved system for access control in a radio communication system would be advantageous and in particular a system allowing for a more accurate resource determination, an improved performance and an increased capacity.
EP1032237 A1 (Motorola) addresses the problem of uplink (subscriber to base station) control, allowing the subscriber to benefit from power control information being sent from multiple base stations and using the best information to limit the power output of subscriber units with impaired uplinks to certain base stations. As such it does not address the current problem of downlink resource allocation for new calls.
WO2000/25444 A1 (Roke) is another uplink scheme to reduce total power output from the subscriber unit during bursts of on/off periods, and does not address the current problem of downlink resource allocation for new calls.
US2002/001292 A1 (Miyamoto) is a downlink power control scheme allowing the minimum power control window to be exceeded on demand within an existing call channel. Consequently it does not address the current problem of downlink resource allocation for new calls.